Process for the preparation of cyclic dicarboxylic acids



United States Patent 3,101,368 PROCESS FOR THE PREPARATION OF CYCLIC DICARBOXYLIC ACIDS Walter Schenlr, Heidelberg, Germany, assignor to Henkel & Cie. G.m.b.H., Dusseldorf-Holthausen, Germany, a corporation of Germany No Drawing. Filed Dec. 28, 1956, Ser. No. 631,058 Claims priority, application Germany Jan. 3, 1956 10. Claims. (Cl. 260-515) This invention relates to a process for producing cyclic carboxylic acids of a desired configuration by a rearrangement of other cyclic carboxylic acids. More specifically, this invention relates to the formation of terephthalic acid by the rearrangement of mixtures of alkali metal salts of other cyclic carboxylic acids.

-It is known that cyclic dicarboxylic acids, e.g. terephthalic acid, are obtained in the form of their metal salts when salts of other cyclic carboxylic acids are subjected to heat treatment, under pressure, if desired, and advantageously in the presence of catalysts. Heretofore the opinion was held that only potassium salts are suitable as starting materials, since they alone gave high yields of the desired cyclic dicarboxylic acids.

I have now found that cyclic dicarboxylic acids or their salts are obtained in yields that are equal to or better than those of the known methods when a mixture of potassium or thallium salts and the corresponding sodium salts of the cyclic carboxylic acids are employed as starting materials.

It is the object of this invention to produce cyclic dicarboxylic acids in improved yield by the rearrangement of a mixture of different alkali metal salts of other cyclic dicarboxylic acids.

Another object is the production of substituted carboxylic acids in improved yield by the rearrangement of different alkali metal salts of substituted cyclic carboxylic acids of a different configuration than that produced. A further object is the use of a sodium salt'of a carboxylic acid in admixture with another alkali metal salt of a carboxylic acid fora rearrangement reaction whereby carboxylic acid salts of a different configuration are produced from the original salts.

These and other objects will become apparent as the description of this invention proceeds.

Cyclic dicarboxylic acids which may be prepared in accordance with this process are, for example, aromatic, cycloaliphatic and heterocyclic dicarboxyl-ic acids containing one or more rings, e.g. terephthalic acid, naphthalene-dicarboxylic acids, diphenylcarboxylic acids, cyclohexaneor cyclopentanedicarboxylic acids and pyridine-dicarboxylic acids. T he acids may contain additional substitutents at the nucleus, e.g. halogen atoms or alkyl groups. clude cyclic acids which contain another salt-forming acid group, e.g. a phenol group, in place of the carboxyl group. Examples of this kind of acids are hydroxybenzoic and hydroxy-naphthalene carboxylic acids.

Cyclic carboxylic acids suitable as starting materials for the rearrangement in the form of their salts are car-.

boxylic acids, e.g. benzoic, phthalic, isophthalic, hemimellitic, trimellitic trimesic, mellophanic, prehnitic, pyromellitic and mellitic acids. Mixtures of benzene-carboxylic acids, e.g. those obtained by oxidizing dialkyl-benzenes, especially crude xylene, with air or nitric acid or by oxidative degradation of higher ring systems, by treatment of carbon-containing substances, e.g. graphite, anthracite,

ligni'te, peat, wood, lignin, coal tar extracts, tars, pitches,

coke or asphalt with nitric acid or oxygen-containing gases Further examples of starting acids are are also suitable. naphthalic, 2-hydroxy-1-napthalene-carboxylic, salicyclrc and diphenic acids.

Furthermore, the term is intended to in- 3,101,308 Patented Aug. 20, 1963 The acids are used as potassium or thallium salts in mixture with their sodium salts, the content of the latter in the mixture varying over wide limits. However, it should not exceed more than 70% since the yields obtained on rearrangement by heating diminish from this point. It is advantageous to use salt mixtures in which the acid with the corresponding quantities of potassium or thallium and sodium hydroxide or carbonate may be used for the rearrangement, the salts being formed during the heat treatment. With the use of diand multibasic acids as starting materials, mixed sodium and potassium salts may also be used successfully. Moreover, in the case of multibasic acids, it is not necessary for the metals to penetrate all of the canboxyl groups, for it is suflicient when only part of the acid groups are neutralized.

The salt mixture is heated in the manner ordinarily employed in the conversion of potassium salts. It is advantageous to work in the presence of catalysts, particularly with the addition of compounds of zinc, cadmium and bivalent iron. Suitable compounds of these metals are, for example, the oxides, halides or salts of metals with organic acids, e.g. the cyclic acids serving as starting materials. Catalysts are added tothe salt mixture in amounts of 0.1-5 weight percent, based on the reaction mixture.

It is advantageous to mix the starting materials thoroughly during the heating, e.g. in a stirring vessel or in a rotary tube oven. The presence of water during heat treatment is undesirable, for it brings about a drop in yield. It is preferable to dry the starting material at 110 C. or higher, advantageously in the reaction vessel itself.

The reaction is conducted at atmospheric or elevated pressure of 1 to 100 atmospheres or more, e.g. at 200 atmospheres or more. The operating pressure may be adjusted by introduction of inert gases or gas mixtures such as carbon dioxide and nitrogen. It is necessary to exclude the presence of oxygen during the reaction.

The temperatures required for the conversion correspond to those known for the conversion of potassium salts, and will generally lie between 300 C. and a temperature at which the starting material will substantially decompose, particularly between 400 and 420 C. How- I ever, hydroxy-carboxylic acids do not require as high ternperatures for their rearrangement, temperatures of 250- carboxylic acids which contain nitro groups and are obtained as by-products of oxidation.

The use of sodium salts together with potassium or thallium salts as starting materials constitutes "an advan tage in that the danger of caking is considerably reduced and any strong increase in temperature during the rearrangement is avoided. This does not cause any drop in- 3 the yields of salts of the desired acids. On the contrary, they are further increased when the composition of the salt mixture is suitable.

From the reaction mixture, after addition of solvents, if desired, the salts may be obtained in the usual manner, or the free cyclic dicarboxylic acids, by means of acidification. It is especially advantageous to follow the method of processing described in copending application Serial Number 626,607, filed December 6,- 1956, now US. Patent 2,930,813.

The following examples are given for purposes of illustration and are not intended to be limitative.

Example I A well-dried mixture of 220 parts dipotassium phthalate and 22 parts disodium phthalate is heated for 4 hours at 410 C., with the addition of 50 parts cadmium oxide catalyst, in a pressure tube under a C pressure of 20 atmospheres. No caking of the reaction material or rise in the reaction temperature are observed. The dark grayto-red product is dissolved in 1,000 parts of hot water and the solution filtered, the catalyst and a little finely-divided carbonaceous residue remaining behind. The clear filtrate containing the organic acid is acidified with a strong mineral acid. The precipitate is suction-filtered and washed with hot water. The filtration residue is boiled in methanol and filtered, the separated terephthalic acid being dried at 130 C. 158 parts terephthalic acid of a high degree of purity is obtained. The purity is established by comparing the infra-red spectrum of a sample pressed with potassium bromide with the corresponding spectrum of a standard sample prepared from pure terephthalic acid and potassium bromide.

Another 2.5 parts benzoic acid is recovered on processing the methanol solution. From the above it can be calculated that 9.1% by weight of disodium phthalate is present. This figure may for simplicity be rounded off to 10%.

Example II A mixture of 148 parts phthalic anhydride, 97 parts anhydrous potash, 37 parts anhydrous soda and 20 parts zinc phthalate which has been predried at 120 C. under vacuum is heated in a shaking autoclave for 5 hours at 415 C. under a carbon dioxide initial pressure of 30 atmospheres. When the autoclave has cooled, the loose, granular, gray reaction product is dissolved hot in 900 parts water, filtered free of a small amount of insolubles, and acidified with mineral acid. The precipitate is washed with a little cold water, then extracted first with boiling water and subsequently with boiling methanol. Upon drying at 150 C., 155 parts terephthalic acid is obtained. The mixed melting point of the ester obtained by esterification of a sample with methanol and pure dimethyl terephthalate showed no depression. The high degree of purity of the resulting terephthalic acid was also demonstrated by infra-red analysis in the manner described in Example I.

Another 6.5 parts benzoic acid and 3.5 parts phthalic acid can be recovered from the aqueous and methanolic solutions. Infra-red analysis shows that these products contain only minute quantities of terephthalic acid.

Example III A mixture of benzenecarboxylic acid prepared from 200 parts crude xylene by preo-xidation with air and secondary oxidation with 30% nitric acid under pressure is adjusted, by means of an alkali solution containing 75 parts caustic potash and 25 parts caustic soda, first to a pH of 6.8, and then, with the use of soda solution, to a pH of 7.2. After 8 parts cadmium phthalate has been added, the mixture is evaporated to dryness and the remaining moisture removed under vacuum. Finally, the gray-red, loose salt is mixed with 5 parts sodium hydride and heated for 4 hours at 425 C. in a pressure vessel under a carbon dioxide pressure of 15 atmospheres.

The reaction product is combined with 1,200 parts water and the small amounts of carbonaceous decomposition products which still contain cadmium compounds are filtered off at C. When the filtrate solution has cooled to 2 C., the terephthalate solution is treated with carbon dioxide at 6 atmospheres. The resulting mixture containing principally acid terephthalate and a little free terephthalic acid is centrifuged. The acid salt mixture which still contains occluded alkali bicarbonate is then treated, with stirring at a boiling heat, with the carboxylic acid mixture produced by the crude xylene oxidation until a sample of filtrate no longer yields any terephthalic acid on addition of a mineral acid. After the terephthalic acid has been separated, repeatedly washed, and dried at 160 C., 226 parts terephthalic acid is obtained.

The Water-insoluble, carbonaceous residue is washed with hot water and treated with hot, aqueous-methanolic initial carboxylic acid solution, whereupon the cadmium compounds dissolve. The insolubles are filtered and thoroughly washed with water and methanol. The filtnate is then combined With the filtrate solution resulting from the separation of terephthalic acid, adjusted to a pH of 7.2 with caustic soda or potash and evaporated to dryness. The resulting salt mixture is then directly subjected to renewed heat treatment for rearrangement into terephthalate.

Example IV A mixture of 280 parts disodium phthalate and 191 parts idithallium-(D-phthalate with 19 parts cadmium phthalate catalyst is dried for 2 hours at 160 C. and subsequently heated in an autoclave for 5 hours under a carbon dioxide pressure of 25 atmospheres. After cooling the autoclave and releasing the pressure the dark-colored slagged reaction product is dissolved in 4000 parts water at a boiling heat. The water-insoluble carbonaceous residue containing cadmium oxide and cadmium carbonate as part of the catalyst present in the mixture is filtered oil and the residue washed repeatedly with hot Water. On addition of hydrochloric acid or another mineral acid the resulting terephthalic acid precipitates in the usual manner from the filtrate solution combined with water. After separation of the terephthalic acid precipitate and subsequent cooling at a temperature of 150 C. 182 parts terephthalic acid is obtained. The degree of purity of the acid is such that it can be employed for esterification without requiring any further treatment.

Example V An aqueous solution of 192 parts potassium lbenzoate and parts sodium benzoate is sprayed in a spray-drier with the addition of an aqueous solution containing 10 parts Zinc benzoate. After heating the salt mixture for 2 hours at a temperature of 180 C. to remove the remaining Water, it is maintained at 435 C. for 3 hours in a pressure vessel equipped with electric heating, under a carbon dioxide pressure of 20 atmospheres. On discharge of the gas mixture in a trap 80.5 parts benzene is obtained through fractional distillation. After the light gray loose reaction product has been dissolved in 1500 parts hot water it is filtered off from the undissolved material land the residue washed Well with hot water. On addition of hydrochloric acid the filtrate in admixture with the washing-water is acidified and adjusted to a pH of 1.8. The terephthalic acid precipitate is subsequently separated from the solution, Washed with hot water and dried at C. 157 parts tereplhthalic acid is obtained.

From the mother liquor another 5.5 parts of unreacted benzoic acid can he recovered which, after conversion into an alkali salt may he employed again in the process of isomerization for producing terephthalic acid.

While I have set forth specific examples and preferred embodiments for the practice of my invention, it will be understood that various modifications may be made without departing from the spirit of my invention and the scope of the appended claims.

I claim:

1. A process for preparing symmetrical aromatic dicarboxylic acids which comprises the steps of forming a starting salt of a non-symmetrical aromatic carboxylic' acid by reacting a material selected from the group consisting of non-symmetrical aromatic carboxylic acids and their anhydrides with a material selected from the group consisting of a mixture of different alkali metal hydroxides, carbonates and oxalates said mixture containing from about to 70% of a sodium compound, heating said formed alkali metal salts in the presence of a catalyst containing a metal selected from the group consisting of cadmium, zinc and bivalent iron, in aninert atmosphere and at a temperature between 340. C. and a temperature at which the starting salts will substantially decompose, whereby said starting salts are converted into the corresponding salts of the symmetrical aromatic carboxylic acid to be prepared, and liberating the free symmetrical aromatic dicarboxylic acid from the product mixtureby acidification.

2. In a process for preparing symmetrical aromatic dicarboxylic acids by heating as a starting material salts of non-symmetrical aromatic carboxylic acids, at a temperature and pressure such. that said starting salts are converted into the corresponding salts of the symmetrical aromatic carboxylic acid to be prepared, the improvement which comprises using a mixture of different alkali metal salts of the said non-symmetrical aromatic carboxylic acids selected from the group consisting of alkali metal and thallium salt as said starting material, one of said salts being the sodium salt which is present as between about 10 and 70% of the mixture.

3. A process for preparing symmetrical aromatic dicarboxylic acids which comprises the steps of heating,

salts will substantially decompose, whereby said benzene carboxylic acid salts are converted into the corresponding salts of terephthalic acid andliberating the terephthalic acid from the product mixture by acidification.

6. A process for preparing terephthalic acid which comprises the steps of heating a mixture of potassium and sodium salts of benzene carboxylic acids in an inert atmosphere and at a temperature between 340 C. and a temperature at which the starting salts will substantially decompose, whereby the said salts undergo a rearrangement reaction and are converted into the salts of terephthalic acid, wherein the amount of sodium salt in the mixture is about 10-50 weight percent, and recovering the terephthalic acid by acidifying the product mixture.

7. A process of preparing terephthalate salts which comprises heating a dry mixture of sodium and potassium benzoate salts containing from about 10 to 70% of the sodium benzoate under 400 to 4000 pounds per square inch pressure of carbon dioxide at temperatures of about 400 to 500 C.

8. A process for preparing terephthalic acid which comprises the steps of heating a mixture of potassium benzoate and sodium benzoate said mixture containing from about 10 to 70% of the sodium compound, in the presence of a catalyst containing a metal selected from the group consisting of cadmium, zinc and bivalent iron in an inert atmosphere and at a temperature between 340 C. and a temperature at which the starting salts will substantially decompose, whereby the starting salts are converted into the dipotassium and disodium terephthalate, and liberating the terephthalic acid from the product mixture by acidifying the product mixture.

9. A process for preparing terephthalic acid which comprises the steps of heating a mixture of potassium and sodium salts of benzene carboxylic acids, wherein said benzene carboxylic acids were obtained from the oxidaas a starting material, alkali metal salts of non symmetrical aromatic dicarboxylic acid, in an inert atmosphere, to a temperature between 340 C. and a temperature at which substantial decomposition of the starting material takes place, and thereafter recovering the symmetrical dicarboxylic acid, wherein the starting material is a mixture of potassium salts of non-symmetrical aromatic carboxylic acids and sodium salts of non-symmettrical aromatic carboxylic acids, said sodium salt comprising fromabout v10 to 70% of the mixture.

4. The process according to claim 3 wherein the starting material contains about 10-50 weight percent of the sodium salts.

5. A process :for preparing terephthalic acid by heating a mixture comprising a material selected from the group consisting of benzene carboxylic acids other than terephthalic acid and anhydrides of these acids, and a material selected from the group consisting of a mixture of different alkali metal hydroxides, carbonates and oxalates wherein one of said alkali metal compounds is a sodium compound which is present as between about 10 and 70% of the mixture, whereby alkali metal-salts of said benzene carboxylic acids are formed in situ, continuing heating of said salts in the presence of a catalyst containing a metal selected from the group consisting of cadmium, zinc, and bivalent iron, in an inert atmosphere and at a temperature between 340 C. and a temperature at which the starting tion of xylene in an inert atmosphere and at a temperature between 340 C. and a temperature at which the starting salts will substantially decompose, whereby the said salts under-go a-rearrangement reaction and are converted into the salts of terephthalic acid, wherein the amount of sodium salt in the mixture is about 10-50 weight percent, and recovering the terephthalic acid by acidifying the product mixture.

10. A process for preparing terephthalic acid which comprises the steps of heating a mixture of dipotassium phthalate and disodium phthalate, said mixture containing from about 10 to of a sodium compound in the presence of a catalyst containing a metal selected from the group consisting of cadmium, zinc and bivalent iron in an inert atmosphere and at a temperature between 340 C. and a temperature at which the starting salts will substantially decompose, whereby the starting salts are converted into the dipotassium and disodium terephthalate, and liberating the terephthalic acid from the product mixture by acidifying the product mixture.

References Citedin the file of this patent UNITED STATES PATENTS 1,937,477 Mills et al. Nov. 28, 1933 FOREIGN PATENTS.

522,829 Belgium Oct. 15, 1953 524,035 Belgium Nov. 30, 1953 1,106,832 France July 27, 1955 

1. A PROCESS FOR PREPARING SYMMETRICAL AROMATIC DICARBOXYLIC ACIDS WHICH COMPRISES THE STEPS OF FORMING A STARTING SALT OF A NON-SYMMETRICAL AROMATIC CABOXYLIC ACID BY REACTING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF NON-SYMMETRICAL AROMATIC CARBOXYLIC ACIDS AND THEIR ANHYDRIDES WITH A MATERIAL SELECTED FROM THE GROUP CONSISTING OF A MIXTURE OF DIFFERENT ALKALI METAL HYDROXIDES, CARBONATES AND OXALATES SAID MIXTURE CONTAINING FROM ABOUT 10 TO 70% OF A SODIUM COMPOUND, HEATING SaID FORMED ALKALI METAL SALTS IN THE PRESENCE OF A CATALYST CONTAINING A METAL SELECTED FROM THE GROUP CONSISTING OF CADINUM, ZINC AND BIVALENT IRON, IN AN INERT ATMOSPHERE AND AT A TEMPERATURE BETWEEN 340*C. AND A TEMPERATURE AT WHICH THE STARTING SALTS WILL SUBSTANTIALLY DECOMPOSE, WHEREBY SAID STARTING SALTS ARE CONVERTED INTO THE CORRESPONDING SALTS OF THE SYMMETRICAL AROMATIC CARBOXYLIC ACID TO BE PREPARED, AND LIBERATING THE FREE SYMMETRICAL AROMATIC DICARBOXYLIC ACID FROM THE PRODUCT MIXTURE BY ACIDIFICATION. 